A senator on Friday asked the FBI to open a perjury investigation into Homeland Security Secretary Kirstjen Nielsen for statements she made to Congress about the administration’s “zero tolerance” policy at the border.

Sen. Jeff Merkley submitted a written request to FBI Director Christopher Wray seeking an investigation into whether Nielsen “committed perjury during her sworn testimony before the House Committee on the Judiciary on December 20, 2018,” when she said the Department of Homeland Security (DHS) did not have a policy of separating families.

The claim was controversial at the time, with Rep. Luis Gutiérrez calling her a liar.

“Calling me a liar are fighting words,” Nielsen said. “I’m not a liar. We did not have a policy of family separation.”

In his letter, however, Merkley said a previously unreleased government memo shows that high-level DHS officials were developing a new policy and legal framework for separating families as far back as December 2017.

Win McNamee / Getty Images

Sen. Jeff Merkley of Oregon.

“This policy, called ‘Separate Family Units,’ was specifically designed to gain media attention and generate a substantial deterrent effect,” Merkley said. “Despite this fact, while testifying under oath before the House Committee on the Judiciary, Secretary Nielsen stated unequivocally ‘I’m not a liar, we’ve never had a policy for family separation.’”

Katie Waldman, a spokesperson for DHS, said the draft memo shows that Nielsen was provided a menu of options to deal with immigration at the border and that the secretary rejected a policy proposal to separate all family units in DHS custody.

Instead, DHS said it followed “the existing decades-long practice” of separating minors from adults when the agency was unable to determine a family relationship, when there is a major risk to the child, or when the parent or legal guardian is referred for criminal prosecution.

“Under zero tolerance, the number of migrants who were prosecuted for entering the U.S. illegally — single adults as well as those part of a family unit — was increased,” Waldman said. “Under this administration, DHS did not issue a new, changed or blanket policy regarding family separations. This has been explained repeatedly to members of Congress.”

In a different memo that was made public through a Freedom of Information Act request in September, Nielsen signed off on the option to prosecute all adults who crossed the border illegally, including those with children, knowing it would lead to family separations.

On Thursday, NBC published a report on a draft of policy options the news outlet had received from Merkley’s office. One option included separating adults from children. Another option was to increase prosecution of parents caught crossing the border.

“The increase in prosecutions would be reported by the media and it would have substantial deterrent effect,” the draft said.

The document and planning would eventually lead to the zero tolerance policy that resulted in the systematic separation of parents from children at the border. Because the parents were charged with illegal entry or reentry and sent to US Marshals custody to face the charges, thousands of children were separated from the adults they were traveling with.

The family separations, which were stopped by a federal judge, resulted in a massive public outcry and backlash last summer. As of December, the Department of Health and Human Services had identified 2,737 separated children who had to be reunited as part of a federal court order.

On Thursday, a government oversight report found that the administration still had no idea how many families it actually had separated. The Office of Inspector General for the Department of Health and Human Services estimated that “thousands” of children may have been separated from their parents since summer 2017, and that the exact number was still “unknown.”